Indoor radon is a major source of radiation for the world's population and may be an important cause of lung cancer. Several studies of populations exposed to domestic radon are being conducted. A meta-analysis of case-control studies of lung cancer and indoor radon yielded results similar to those based on studies of underground miners, for whom radon has been established as a human carcinogen. Case-control studies of lung cancer and radon among residents living in underground dwellings in China, and among smoking and non-smoking females in Missouri are in the field or in the data analysis stage. A large case-control study of childhood leukemia provided no evidence of an association between indoor radon and childhood acute lymphocytic leukemia. Studies of populations exposed to fallout from nuclear tests or accidents are another major focus of this project. The Chernobyl nuclear reactor accident in Ukraine has led to an unprecedented increase in thyroid cancer among children in Belarus, northwestern Ukraine, and bordering Russian provinces. With the assistance of the Department of Energy (DOE) and the Nuclear Regulatory Commission (NRC), NCI has organized long-term follow-up studies of about 15,000 children in Belarus and 30-50,000 in Ukraine. Villagers living along the banks of the Techa River in Russia were exposed to chronic external and internal radiation from the Mayak nuclear facility. Cancer mortality is being evaluated in a cohort of approximately 28,000 people who received large doses of radiation from the radioactive waste dumped into the river. Preliminary data on a cohort of 20,000 residents of villages near the Semipalatinsk Atomic Bomb Test Site in Kazakstan suggest that this cohort might be informative about cancer risks associated with exposure to radioactive fallout. REB plans to evaluate radiation dosimetry estimates, and conduct a thyroid screening study of residents of heavily exposed and lightly exposed villages who were under 15 years of age at exposure.